A few years ago I completely rebuilt my bus's entire cooling system to address some worsening overheating when in hot weather: I replaced the radiator core, fan, fan motor, hydraulic/PS pump, Danfoss fan speed valve, both thermostats/seals/gaskets, and sealed all air gaps in the shroud. I also installed a separate transmission cooler to draw off some heat before the fluid reached the engine's heat exchanger, and I checked the water pump's impeller. I use coolant with slightly more water than coolant mix to also help heat transfer. The engine temps are still not quite where I want them to be, requiring me to drive very carefully and slowly in hot weather. When I mentioned this to the mechanics where my bus is having its camshaft issue(s) fixed, I was told to change the thermostats to a lower temperature rating to aid cooling. The thermostats in the engine now are Weir Stat 180F, the same as the factory originals, and they open fully at about 195F, the same temperature that the fan goes to full speed.
I don't know whether to believe the mechanic about cooler t'stats helping to cool the engine better. Surely cooling is determined by the radiator's heat rejection ability, not the t'stats' temperature rating? If instead I put in 160F t'stats the coolant would begin flowing through the radiator sooner, but by 195F there would be no difference compared to 180F t'stats. Is it true that cooler t'stats will help the overall cooling, or will they only start the cooling process sooner with little to no effect on the engine's maximum temperatures? Or, is there any downside to using 160F t'stats? Now would be the time to change them if there's any worthwhile improvement, but Detroit put in 180F t'stats for good reason, so I'm hesitant to change what Detroit's engineers decided works best.
I know this subject of t'stat temperatures doesn't have simple clearcut answers, and the internet is full of anecdotal stories and unverifiable 'facts' about engine cooling. What are the real-world experiences of folk here with the t'stats in their rear-engined two-strokes?
Many thanks, John
You are fine with 170 or 175 t stats, Detroit's min operating temp is 170 degrees and your version of DDEC doesn't use any temperature sensitive sensors.
The engineers that used to participate in the online hobby ALL back your understanding of cooling system design.
A lower thermostat only gives you a somewhat longer run at reaching the same overheating condition.
Anecdotes are proven out, when the subject reaches the top of his local hill before it boils overn when before he only made it most of the way up. Problem delayed, not solved.
Heat rejection limits cannot be denied.
Happy coaching!
Buswarrior
I have always run 170 degree thermostats in my 6v92TA but also have the shutters open sooner and the fan engage to full on sooner. I watch the temps very closely and very rarely exceed 195. I travel about 65 mph and pull a 5000 lb truck.
I have always run 170 degree thermostats in my 6v92TA but also have the shutters open sooner and the fan engage to full on sooner. I watch the temps very closely and very rarely exceed 195. I travel about 65 mph and pull a 5000 lb truck. I also added an extra cooler for the trans. It is a small rad type mounted with the rad so the fan cools both.
He needs to change the wax( Danfoss) valve to come on high at 185 degrees starting the rejection process earlier before it reaches 195 degrees. I am not a fan of the Sauer/Danfoss wax valve, my Country Coach used one and it wasn't accurate all the time ,I change mine to a brand I forgot the name of now.He will be fine if he the starts the rejection process before 195 degrees,and all it will cost him is a little fuel mileage. And he is good with 170 degree t stast 's and have the fan come on at 185 degrees.I think his bus was spec for Northern AZ where you need heat
Quote from: luvrbus on March 11, 2023, 07:21:10 AM
He needs to change the wax( Danfoss) valve to come on high at 185 degrees starting the rejection process earlier before it reaches 195 degrees. I am not a fan of the Sauer/Danfoss wax valve, my Country Coach used one and it wasn't accurate all the time ,I change mine to a brand I forgot the name of now.He will be fine if he the starts the rejection process before 195 degrees,and all it will cost him is a little fuel mileage. And he is good with 170 degree t stast 's and have the fan come on at 185 degrees.I think his bus was spec for Northern AZ where you need heat
The Danfoss valve is a 7WA110-2 solenoid directional control valve that diverts some of the fan motor's hydraulic fluid back to the reservoir below 195F to make the fan run at half speed, and this valve is actuated by a 195F temperature switch on the passenger-side thermostat housing. I've fitted an override switch to allow the fan to run all the time at full speed in the summer, and this slightly helps with engine temperature unless it's a long hill, in which case the temps still creep up to 200F or very slightly higher. The radiator is a 6-row copper/brass, now with 73 rows of dimpled tubes instead of the original's 57 rows of plain tubes, and now has much greater fin count between tubes; it's also about 9% greater surface area than the original's, which should also result in better cooling. I moved the hydraulic fluid cooler to a different location to now allow unobstructed airflow to the radiator, and the 9-blade Multiwing fan moves noticeably more air than the original 6-blade metal fan that made more noise than airflow. The new fan's rotation is even the opposite direction of the original fan's to produce a smoother airflow out the bottom of the shroud!
I can't think of anything else I can do to reduce the temperatures except to add a second radiator on the opposite side. A top-flow pickup truck radiator will fit there, and there's space for four 12" or 14" electric fans on a shallow shroud; I can add a second coolant booster pump to divert coolant from the 1" pipes that feed the heaters/defroster, and the new radiator's coolant can then rejoin the coolant out the bottom of the main radiator before the water pump. The present heaters/defroster are rated at a total of 150,000 BTU/hour, and I imagine a new radiator will have much greater heat rejection than that; when I turn on the heaters the engine temperature drops by a few degrees, so I should get a useful reduction in engine temp with this new radiator.
My bus was built for Mid-Placer Public Schools in the foothills of the California Sierras, but it wasn't fitted with sanders or automatic tire chains or a third heater, so I'm guessing it was built for normal CA temperatures, not Flagstaff-cold! The flat-land Crown Super IIs all have the 253HP 6V92TAC, and they don't overheat even when climbing long 6% grades in hot weather; my bus has the 277HP engine, but its radiator is the same size as the 253HP buses' radiators. I'm thinking that my radiator is simply slightly too small, so adding a few more square feet of radiator to compensate for that may be the best solution. And yes, I made a radiator mister system that does almost nothing to reduce temperatures!
Thanks, John
I wonder if you just hit the nail on the head. Undersized rad. Dimpled tubes are the way to go but with the smaller rad sounds like it just be the issue. Mine is a 277 hp DDECIV 6V92 TA and I had to go to the 8v71 rad with extra rows of tubes. I ordered dimpled tubed but the rad place screwed up and made it with the normal tubes but more rows and more fins. Seems to work fine but when it comes time to replace it I will definitely be getting a dimpled tube one.